Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTSs) and a plurality of subscriber units, often termed mobile stations (MSs).
Wireless communication systems are distinguished over fixed communication systems, such as the public switched telephone network (PSTN), principally in that mobile stations move between BTS (and/or different service providers) and, in doing so, encounter varying radio propagation environments.
In a wireless communication system, each BTS has associated with it a particular geographical coverage area (or cell). The coverage area is defined by a particular range where the BTS can maintain acceptable communications with MSs operating within its serving cell. Often these cells combine to produce an extensive coverage area. The preferred embodiment of the present invention is described with reference to the Third Generation Partnership Project (3 GPP) defining portions of the Universal Mobile Telecommunication Standard (UMTS), including the time division duplex (TD-CDMA) mode of operation.
In UMTS parlance, a BTS is referred to as a Node B, and subscriber equipment is referred to as user equipment (UE). With the rapid development of services provided to users in the wireless communication arena, UEs encompass many forms of communication devices, from cellular phones or radios, through personal data accessories (PDAs) and MP-3 players to wireless video units and wireless internet units.
In wireless communication parlance, the communication link from the Node B to a UE is referred to as the downlink channel. Conversely, the communication link from a UE to the Node B is referred to as the uplink channel.
In such wireless communication systems, methods for simultaneously utilising the available communication resource exist where such communication resources are shared by a number of users. These methods are termed multiple access techniques. Typically, some communication resources (say communications channels, time-slots, code sequences, etc) are used for carrying traffic whilst other channels (which may be logical or dedicated channels) are used for transferring control information, such as call paging, between the Node Bs and the UEs.
It is worth noting that transport channels exist between the layer 1 and the medium access control (MAC) in the system hierarchy. Transport channels define ‘how’ data is transferred over the radio interface. Logical channels exist between MAC and the radio link control (RLC)/radio resource control (RRC)layers. Logical channels define ‘what’ is transported. Physical channels define what is actually sent over the radio interface, i.e. between layer 1 entities in a UE and a Node B.
A number of multiple access techniques exist, whereby a finite communication resource is divided into any number of physical parameters, such as:                (i) Frequency division multiple access (FDMA) whereby the total number of frequencies used in the communication system are shared,        (ii) Time division multiple access (TDMA) whereby each communication resource, say a frequency used in the communication system, is shared amongst users by dividing the resource into a number of distinct time periods (time-slots, frames, etc.), and        (iii) Code division multiple access (CDMA) whereby communication is performed by using all of the respective frequencies, in all of the time periods, and the resource is shared by allocating each communication a particular code, to differentiate desired signals from undesired signals.        
Within such multiple access techniques, different duplex (two-way communication) paths are arranged. Such paths can be arranged in a frequency division duplex (FDD) configuration, whereby a frequency is dedicated for uplink communication and a second frequency is dedicated for downlink communication. Alternatively, the paths can be arranged in a time division duplex (TDD) configuration, whereby a first time period is dedicated for uplink communication and a second time period is dedicated for downlink communication.
Present day communication systems, both wireless and wire-line, have a requirement to transfer data between communications units. Data, in this context, includes signalling information and traffic such as video and speech communication. Such data transfer needs to be effectively and efficiently provided for, in order to optimise use of limited communication resources.
In TDMA cellular communication systems (e.g. GSM (Global System for Mobile Communications) systems) and combined TDMA/CDMA cellular communication systems (e.g. UMTS systems), time division duplex (TDD) is employed to divide the allocation of signals for uplink transmission and downlink transmission. For each consecutive TDMA frame of a given frequency channel, some timeslots are allocated to uplink communication, and some are allocated to downlink communication.
The deployment of cells conforming to the Third Generation Partnership Project (3 GPP)/UMTS time division duplex (TD-CDMA) mode of operation usually assumes that large groups of cells (and in the limit the whole network) co-ordinate the split of uplink and downlink assigned slots so that the switching points in time (uplink to downlink or vice versa) are the same across this group of cells. Without this, near-located cells could severely interfere with each other because uplink and downlink data transfer would be attempted at the same time on the same frequency and timeslot.
In some cellular communication systems, a user can be assigned a given radio bearer according to his or her specific request for service. The data rate (also termed bandwidth) provided can be lower or higher depending on the service or usage being requested. Thus in UMTS, for example, higher data rate users may be assigned to a dedicated traffic channel, whereas lower data rate users may be assigned to an inferior channel alternative, for example a combination of Random Access Channel (RACH) and Forward Link Access Channel (FACH), hereinafter referred to as a RACH/FACH combination. One disadvantage associated with the RACH/FACH combination is that power control operates less efficiently than in dedicated channels.
In the 3 GPP standard, dynamic re-use of the limited communication resource is a major factor in providing for efficient and effective communications. In order to dynamically re-use the resources available, the concept of shared channels has been further developed.
The current proposal in 3 GPP is for an uplink resource to be requested on a random access channel (RACH). A channel (communication resource) will be granted by the system/network infrastructure on a forward access channel (FACH). A packet-data transmission would then begin using a dedicated channel (DCH). The procedure would be similar if a DCH is reactivated after a break in transmission.
It is known that shared channels can be used when the UE has been allocated a dedicated channel (DCH), i.e. in UMTS parlance it is in a cell_DCH state. Allocations of shared channels are indicated from a UMTS terrestrial radio access network (UTRAN) using the PHYSICAL SHARED CHANNEL ALLOCATION message that can be mapped to the ‘logical’ dedicated control channel (DCCH) or the SHCCH. Such a use of a logical DCCH or a transport-format SHCCH indication allows the UE to be allocated a communication resource by transmitting a PHYSICAL SHARED CHANNEL ALLOCATION message on the downlink of its DCH. The SHCCH is predefined as a mapped RACH or an uplink shared channel (USCH) i.e. a transport channel, in the uplink.
When uplink shared channels are employed, the UE still sends a request for a communication resource to the infrastructure. This request is termed a (physical uplink shared channel) PUSCH CAPACITY REQUEST message. The PUSCH CAPCITY REQUEST message is mapped, within the system infrastructure to a shared control channel (SHCCH), which is a ‘logical’ channel within the communication system.
The inventor of the present invention has recognised the inconsistencies between the methods for requesting and allocating of communication resources, particularly in relation to the TDD mode of operation in the 3 GPP standard and when a UE is in a cell_DCH state. In particular, the PUSCH CAPACITY REQUEST message is undesirably limited to only using the SHCCH logical channel. This is inefficient as it means that a random access channel (RACH) must be used for PUSCH CAPACITY REQUEST messages when alternative, more efficient resources could be utilised.
A need therefore exists for an improved communication system, communication unit and method of requesting a communication resource wherein the abovementioned disadvantages associated with prior art arrangements may be alleviated.